Beginning in the early 1980s, new technologies, began to permit evaluation of the expression of individual genes. Recent technological advances have expanded those evaluations to permit the simultaneous detection of the expression of tens of thousands of genes and to support holistic evaluations of the entire genome. The application of these technologies has enabled researchers to unravel complexities of cell biology and, in conjunction with toxicologic evaluations, the technologies are used to probe and gain insight into questions of toxicologic relevance. As a result, the use of the technologies has become increasingly important for scientists in academia, as well as for the regulatory and drug development process.